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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

You’ll find the sheet shown in Figure 3.4 very valuable, because it lists every block size you can use for a network address. Notice that the block sizes are listed starting from a block size of 4, all the way to a block size of 128. If you have two networks with block sizes of 128, you’ll quickly see that you only can have two networks. With a block size of 64, you can only have four networks, and so on, all the way to having 64 networks if you only use block sizes of four. Remember that this takes into account that you are using the command ip subnet-zero in your network design.

Now, just fill in the chart in the lower-left corner, and then add the subnets to the worksheet and you’re good to go.

So let’s take what we’ve learned so far about our block sizes and VLSM table and create a VLSM using a Class C network address 192.168.10.0 for the network in Figure 3.5. Then fill out the VLSM table, as shown in Figure 3.6.

2 hosts Network E

2 hosts Network G

Lab_A

2 hosts

Lab_B

F0/0

Network H

F0/0

Lab D

2 hosts Network F

Lab E

_

_

We still have plenty of room for growth with this VLSM network design. We never could accomplish that with one subnet mask.

In Figure 3.5, we have four WAN links and four LANs connected together. We need to create a VLSM network that will allow us to save address space. Looks like we have two block sizes of 32, a block size of 16, and a block size of 8, and our WANs each have a block size of 4. Take a look and see how I filled out our VLSM chart in Figure 3.6.

192.168.10.108/30

FIGURE

192.168.10.100/30

192.168.10.112/30

6 hosts Network D

192.168.10.8/29

14 hosts Network A

192.168.10.16/28

192.168.10.32/27

192.168.10.64/27

30 hosts Network B

F0/0

192.168.10.104/30

3.5

A VLSM network, example one

F0/0

20 hosts Network C

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